1. Field of the Invention
The present invention relates to a polymer solid electrolyte that is useful for primary cells and secondary cells, and more particularly to a self-crosslinked polymer solid electrolyte that is suitable for film-like polymer cells. The present invention also relates to a composite solid electrolyte composed of the self-crosslinked polymer solid electrolyte and an electrically insulating material, as well as to a thin solid cell utilizing the composite solid electrolyte. Further, the present invention relates to a block-graft copolymer used for manufacture of the self-crosslinked polymer solid electrolyte.
2. Description of the Related Art
As solid electrolyte, inorganic materials such as .beta.-alumina, Li.sub.2 TiO.sub.3, RbAg.sub.4 I.sub.5, AgI, and tungstophosphoric acid have been developed and widely known. However, inorganic materials have drawbacks of 1) large specific gravity, 2) difficult in formation into a desired shape, 3) impossibility of obtaining soft and thin film, and 4) low ion conductivity at room temperature, which cause problems in practical use.
Recently, organic materials have become of interest as potential material that can mitigate the above-described drawbacks. Such polymer solid electrolyte is manufactured in a manner such that electrolyte (mainly inorganic salt), such as LiClO.sub.4 or LiBF.sub.4 --which serves as a carrier--is mixed and dissolved into a matrix polymer such as polyalkylene oxide, silicone rubber, fluororesin, or polyphosphazen. Such polymer solid electrolyte is lighter and more flexible than inorganic materials, and therefore can be easily machined or formed into film. Under such circumstance, active research and development have been conducted in recent years in order to obtain a polymer solid electrolyte that exhibits higher ionic conductivity while maintaining the above-described characters.
A presently known most effective method of imparting high ion conductivity is a technique in which aprotic organic electrolytic solution is absorbed into a polymer solid electrolyte in any manner in order to obtain solid electrolyte in the form of gel (see M. Armand, Solid States Ionics, 69, pp. 309-319 (1994)). Polymers that are usable as a matrix in the gel-type solid electrolyte are generally divided into 1) linear polymers such as polyether polymers and fluororesins, and 2) crosslinked polymers such as polyacrylic polymers.
Applications of the above-described linear polymers are shown in, for example, I. E. Kelly et al., J. Power Sources, 14, pp. 13 (1985) and U.S. Pat. No. 5,296,318. However, both cases have the problems of leakage of electrolytic solution from polymer and insufficient strength of film. Further, since electrolytic solution acts as a plasticizer for polymer serving as a matrix, the polymer itself dissolves into the electrolytic solution even when the temperature of the system increases slightly.
For the crosslinked polymers, there have been proposed a method in which a liquid monomer mixed with an electrolytic solution is polymerized to yield a crosslinked polymer including an electrolyte (see PCT/JP91/00362, International Laid-Open No. W091/14294). However, in this method, when the crosslinking degree of the polymer is increased, the ion conductivity decreases considerably, and when the crosslinking degree is decreased, the solid strength (elastic modulus) of the polymer decreases, so that film having a sufficient strength cannot be obtained.
In Japanese Patent No. 1842047 (Invention A), the applicant of the present invention proposed a block-graft copolymer, which is a model of the present invention and the method of manufacturing therefor. Also, in Japanese Patent No. 1842048 (Invention B), the applicant of the present invention proposed a polymer solid electrolyte composed of a block-graft copolymer composition in which in order to increase the ion conductivity of the block-graft copolymer, there is mixed an inorganic salt containing at least one element selected from the group consisting of Li, Na, K, Cs, Ag, Cu, and Mg in an amount of 0.05-80 mol % with respect to the alkylene oxide unit thereof.
In Japanese Patent Publication (kokoku) No. 5-74195 (Invention C), the applicant of the present invention proposed a Li cell which includes, as an electrolyte, a composite material composed of a Li ion salt and a block-graft copolymer similar to the above. Further, In Japanese Patent Application Laid-Open (kokai) No. 3-188151 (Invention D), the applicant of the present invention proposed a block-graft copolymer composition which is obtained by adding polyalkylene oxide to the above-described inorganic ion salt composite of a block-graft copolymer.
In the inventions B, C, and D, an organic solvent is added, together with an inorganic salt or the like, to a resultant block-graft copolymer in order to dissolve it, and after formation, the organic solvent is removed through drying in order to yield a polymer solid electrolyte. However, since all the polymer solid electrolytes is slightly low in terms of ion conductivity, they have not come into practical use.
In order to improve the ion conductivity, in Japanese Patent Application Laid-Open (kokai) No. 7-109321, the applicant of the present invention has proposed a composite solid electrolyte in which a nonaqueous electrolyte including a cyclic carbonate solvent and an inorganic salt is included in the same block-graft copolymer as that described above. Although the invention improved the ion conductivity, and at the same time increased the film strength drastically, it was found that if such a composite solid electrolyte is applied to household small cells in which characteristics at low temperatures (room temperature to -20.degree. C.) are regarded as important, satisfactory lower temperature characteristics are difficult to be obtained because of the high viscosity and the high melting point of the cyclic carbonic acid ester. This problem necessitates adding, as a secondary component, a large amount of a low boiling-point linear ester or a carbonic acid ester, which is a generally known method for improving the low-temperature characteristics of cells. However, since these solvents are good solvents for the above-described block-graft copolymer, addition of a large amount of such a solvent causes dissolving of the polymer solid electrolyte itself.
Further, when the above-described composite solid electrolyte is applied to large-sized cells for use in electric vehicles and electrical power leveling systems and the like for operation at high temperatures (60-80.degree. C.), which cells are expected to come into practical use in the future, polyalkylene oxide having low vapor pressure is optimally used as a main component. However, even in this case, use of a large amount of polyalkylene oxide causes swelling and dissolving of the polymer solid electrolyte.